"Pimples during the teen years are primarily due to the huge surge in hormones during puberty," says New York City–based Marsha Gordon, M.D., a consulting dermatologist to St. Ives. "The hormones increase sebum production, making the cells that line your pores sticky—which in turn clogs them so that oil can't escape." When oil gets backed up, your pore develops a bump. If it's squeezed or becomes too full and the lining cracks, sebum and bacteria seep out into the skin and provoke inflammation. Chicago-based dermatologist Brooke Jackson, M.D., adds, "If you're involved in sports that require gear like helmets or pads, trapped sweat and dirt can trigger breakouts. It's important to wash your face as soon as possible after workouts."

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"Establishing a good routine in your teens may prevent problems in your 20s," Jackson says. "Your doctor can prescribe medications, and you may buy products to help, but they don't do any good if you don't use them regularly." The number one mistake girls make in taking care of their skin? "Falling asleep with makeup on!" Gordon answers. "Leaving your cosmetics on overnight can cause breakouts." It's also crucial to give your skin the occasional scrub or peel. "Eoliation is essential to slough off dead skin cells," she notes. "But be gentle. Start mild and increase frequency once you know what your skin is able to handle."

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"Skin needs to be able to breathe," says New York City celebrity facialist Tracie Martyn. "Stay away from anything too waxy or greasy when it comes to makeup or facial products." Look for moisturizers with salicylic acid—they hydrate as they unclog pores and gently eoliate skin. And even if you have oily skin and are prone to breakouts, moisturizing is still a must. "It's a myth that drying out a zit will make it go away," Gordon says. "Overdrying may actually inflame a pimple and slow down recovery."

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To avoid infections and scars, don't take matters into your own hands (regardless of how tempting it may be). "I've had teen clients who have attacked their spots with needles, pens, and tweezers," says Wanda Herrera, a cosmetologist at Cal-A-Vie spa in Vista, California. "They've ended up with scars that can take months, if not years, to heal." If your spot doesn't fade on its own within a few days, see a skin-care professional.

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Healthy skin starts from the inside out: "Eat a diet rich in good fats—found in foods like salmon, avocado, nuts, and olive oil—for a soft, supple, and acne-free face," says Nicholas Perricone, M.D., a Meriden, Connecticut–based dermatologist. "Sugary or starchy foods [like fries or white bread] can exacerbate problems." Also remember to get enough sleep. "Get at least eight hours every night. It's vital to avoid puffy eyes and dark circles and to be vibrant!" he notes.

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"Don't ever leave your house without sunscreen. Use at least SPF 30 to protect against age spots and skin dehydration—not to mention cancer," advises Beverly Hills aesthetician Sonya Dakar. "And if you think being in the sun will make your acne go away, think again. Acne-causing bacteria love the sun and sweat; the sun 'ripens' the breakout and makes it worse."

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"Inhaling just one puff of a cigarette generates more than a trillion free radicals in your lungs, which triggers an inflammatory response in all organs, including the epidermis," Perricone says. "Tobacco constricts blood vessels, which reduces blood flow to the skin and results in a gray, lifeless, and unhealthy-looking complexion, as well as leathery skin and wrinkles in the long term."